The present invention relates to an exhaust-gas cleaning device for an engine capable of decreasing hydrocarbons (HC) contained in engine exhaust gases, the exhaust-gas cleaning device comprising an HC-catalytic converter which adsorbs and converts HC.
An example of a conventional air-fuel ratio control device for an internal combustion engine is disclosed in Japanese Unexamined Patent Publication No. 11-82111, in which an HC-catalytic converter including an HC-adsorbing material and a three-way catalyst layer formed on top of the HC-adsorbing material is placed in an exhaust passage, the air-fuel ratio control device having an air-fuel ratio controller for controlling the air-fuel ratio of an intake air-fuel mixture of the internal combustion engine in such a manner that the air-fuel ratio of exhaust gas at a downstream end portion of the HC-catalytic converter is increased (air-fuel mixture gas becomes “lean”) by a specified amount during release of HC from the HC-adsorbing material. In this air-fuel ratio control device, the air-fuel ratio at the downstream end portion of the HC-catalytic converter is controlled to increase by the specified amount during release of HC taking into account the difference between a rate at which HC released from the HC-adsorbing material dissipates into the three-way catalyst layer and a rate at which oxygen contained in the exhaust gas is taken into the three-way catalyst layer, so that at least such an amount of oxygen that is necessary for oxidizing the released HC is adsorbed on the surface of the three-way catalyst layer and HC released from the HC-adsorbing material is converted.
In the aforementioned air-fuel ratio control device which is so constructed as to control the air-fuel ratio of the intake air-fuel mixture of the internal combustion engine such that the air-fuel ratio at the downstream end portion of the HC-catalytic converter is increased (air-fuel-mixture gas becomes “lean”) by the specified amount during release of HC from the HC-adsorbing material, it is possible to decrease HC released from the HC-adsorbing material by causing it to react with oxygen contained in the exhaust gas if a catalytic element provided in the HC-catalytic converter is activated. It is to be noticed, however, that although the HC-adsorbing material begins to release HC at temperatures of about 150° C. to 200° C., the catalytic element of the HC-catalytic converter is activated at about 250° C. or above even when a low-temperature-activation catalyst is used. This produces a problem that HC released from the HC-adsorbing material before activation of the catalytic element cannot be decreased by causing sufficient reaction with oxygen in the exhaust gas.